Method of anodically polishing stainless steel



Patented Jan. 9, 1945 METHOD or ANonrcALLvrousmNG STAINLESS STEEL v charles L. Faust, columbus, ohio, assigner t Battelle Memorial Institute,

corporation of Ohio Columbus, Ohio, a

Application November 6, 1939, Serial No. 302,950

v3 Claims. (Cl. 2414-140) This invention relates te e method ef and ein veieetielyte for enedieeuy polishing stainless stee1.

More particularly the invention relates to an -aqueous electrolyte comprising phosphoric acid,

chromic acid and water and to the use' of such 1 electrolytein the anodic polishing of stainless steel. v

Various compositions of electrolyte have heretofore been proposed for use inthe anodictreat'- ment of stainless steel. With the exception, however, of the electrolytes disclosed and claimed chromic acid and water for compositions of electrolytes coming within the scope of my invention.

In the accompanying diagram, the'respective sides Vof the triangle indicateV the percentages of phosphoric acid (HaPOi), of water `(H2O), and of chromic acid (Crm) from 0 to 100%. Onthe basisoi experimental data, I havedetermined the relative proportions of phosphoricacid','chro mic acid and water that give compositions of elecin my copendingV application Serial No. 218,388,

filed July 9, 1938, most of the compositions proposed have proved unsatisfactory for effecting hghlylustrous, mirror-likenmshes on stainless steel. f

By the term fstainless steel, it is meant to include straight chromiu'mand chromium-nickel ferrous alloys, having a minimum chromium content of about 12% and generically-known as stain- .less steels, stainless irons or rustless irons. I have now found that an aqueous electrolyte comprising phosphoric acid, chromic acid and water, as the essential ingredients, can be satisfactorily used vin the' anodic polishing of stainless steel.

If the relative proportions of these essential ingradients of the bath be kept within certain limits, which I have determined, very satisfactory polishes can be produced in the case of stainless steel, while at the same time producing surfaces I having relatively superior characteristics to those obtainable by mechanical polishing or bufling operations.

It is therefore an important object of my invention to provide an electrolyte containing phosphoric acid, chromic acid and water within certain limits as to their relative proportions', for use in the anodic polishing of stainless steel to impart thereto-a highly lustrous surface.

It is afurther importantI object of this invention to'provide a method of anodically polishing stainless steel and the like using an electrolyte of novel composition that is operative throughout a wide range of anode current densities and temperatures. A

Other and further important objects of this invention will become apparent from the Adis closures in the specication and the accompanying drawing. This invention (in its preferred form) is-illustrated in the, drawing and hereinafter more fully described. c e v On the drawing: j The' figure represents a triaxial diagram showing` therelative of phosphoricv acid,

kil() trolyte that are operative for the anodic polishing of stainless steel. The area representing operative compositions of electrolyte isvdened on the accompanying diagram by the solid lines AB, BC, CD and DA.- Within the area so defined, anyA composition selected will be found to be operative in the method hereinafter described for the anodic polishing of stainless steel.

In order to get the best polishing results, how? ever, I have found that the range of proportions of phosphoric acid, chromicacid and water should be kept Within somewhat narrowery limits and these narrower limits are represented on the accompanying diagram bythe area defined by the lines AB, BF, dot and dash line FE and line 4EA. The preferred compositions of electrolyte, with :respect to the relative proportions of phosphoric acid, chromic acid and water, lie-within this sec- `ond area, which is wholly enclosed within the broader area rst defined. A

The reading of a triaxial diagram such as the accompanying one is well understood but the following will be given for purposes of illustration. The point on thediagram represented by Ythe letter `A, for instance, indicates a composition comprising a small but significant proportion, say y0.1% of phosphoric acid." 65% of Jchromic acid and the balance, or slightly less than 35%, water; the point indicated by the reference letter B a composition comprising" 85% of phosphoric acid,

a. small but signicant proportion, say 0.1% of chromic acidand the balance, or slightly. less I than 15%, water; the point indicated by the reference letter C a composition comprising 44% of phosphoricacid, say 0.1% of chromic acid and D a composition comprising say 0.1% of phosphoric acid, 34% of chromic acid and the balance,

somewhat less than'4 66%, water. The maximum phosphoric acid content is 85% and the minimum about 0.1%: the maximum chromic acid y content 65% and the minimum about 0.1%; and

the maximum water content about 66% and the minimum about 13%. lT he preferred. composi- I6 tion limits are from '6.1 to 85% phosphoric acid,

from 0.1 to 65%'chromic acid, and from 13 to 47% water. u

While the triaxial diagram shows the relative, proportions of phosphoric acid, chromic acid and water in a system consistingvof only these three components, suitable baths of these three ingredients may also include other ingredients, such as other acids and/or salts. From the diagram, however, the relative proportions that phosphoric acid, chromic acid and water should bear to each other may-be determined for operative and preferred ranges of composition, regardless of what other non-essential ingredients may be present in the bath. l

Thus, fooi` instance, in making up an elect'rrfv lyte for the anodic polishing of stainless steel, one might select the point X on the triaxial diagram as being a bath of lpreferred composition. The composition represented by the point X would be 56% phosphoric acid, 12% chromic acid and 32% water. During the continued use of such a bath in the electropolishing of stainless steel, the bath composition would necessarily change, owing 'to the' anodic dissolution intothe bath of chromium, nickel and iron from the stainless steel undergoing polishing. There might also be some change in the water content, such as an increase due to absorption by the bath of moisture from the air, or a decrease due to evaporation of wa'r from the surface of the bath, to decomposition of water-by electrolytic action, or to loss of water from the rbath by en trainment thereof in gases given off from the bath.

Notwithstanding such changes -in its composition as may occur during continued use, if the relative percentagesof phosphoric acid, chromic acid and water, expressed as percentages by Weight of the Atotal weight ofv only these three ingredients in the bath composition, remain within the area dened by the lines AB, BF, Fiji and EA, the bath will continue to operate satisfac torily. Even if the bath composition be so altered during continued operation that itl falls within A the less preferred area dened by the lines EF, FC, CD and DE, on the accompanying diagram, the bath will continue to function, although not so satisfactorily.

Consequently', where the relative percentages of phosphoric acid, chromic acid and water in a given bath composition lie within either the preferred or less preferred areas defined on the accompanying triaxial diagram, such bath com.- position is intended to come within the scope of my invention, even though it may contain other acids .than phosphoric and chromic and even though it may contain a substantial quantity of metallic salts.

Instead of chromic acid, soluble chromates and bichromates may -be substituted thereforand are to be considered the equivalent of chromic acid oil a stoichiometric basis. The term chromic acid equivalent, as used in this specification and in the claims, is therefore intended to include .chromic acid `itself (CrOa) and stoichiom'etrically a composition indicated as suitable by reference to the 'accompanying triaxial diagram, the stainless steel, or an article .having a surface of stainless steel, is made the anode in a bath of the selected composition and an electric current is passed therethrough of suicient density and for a suicient length of time to produce the desired high degree of luster, or polish, on the metal surface. By employing an electrolyte having a composition within the preferred area defined on the accompanying triaxial diagram, a highly lustrous, mirror-like surface can be readily obtained.

The highly lustrous surface obtainable` by my method, using an electrolyte of preferred composition, is an important feature of my invention and one that sharply distinguishes it vfrom prior art iinishes produced' in the electrolytic cleaning of stainless steel.

The formation of' highly polished and lustrous surfaces is undoubtedly associated with the presence 'of a polarizing lm over the surface of the metal during the process of anodic dissolution. The nature of this film is such that selective attack on -the various phases present in the stainless steel, rustless iron or the' like, is minimized. Anodic dissolution'. apparently takes place at a relatively high rate and at a high anodic polariza- -tion value, with the result that the anodic dissolution of the metal acts to levelthe crystal -surfaces thereof and to produce a mirror-like finish. These conditions do-not prevail inthe simple electrolytic cleaning treatment known to the prior art.

In order to obtain the best results in a reasonablelength of time, it is preferable to `use relatively high currentdensities, such as those of the order of magnitude of from -100 to 1000 amperes equipment. The length of time to effect the desired results depends upon the magnitude of the vcurrentliensities employed and .to some extent upon the particular analysis of the stainless steel, rustless iron or the like to be polished, and also upon Athe character of the surface of the alloy initially. Rough surfaces, of course, require a longertime to polish than relatively smooth ones.

With any'of the compositions of electrolyte lying within the 'preferred area defined by the .linesv AB, BF, FE and EA, on the'accompanying triaxial diagram, excellent polishes are obtained on 18-8 chromium-nickel, 24--12 chromiumnickel, and straight chromium stainless steel, when the stainless steel is made the anode therein at current densities of 500 amperes per sq. ft. and the treatment carried out for a period of 11/2 to 12 minutes, employing a bath temperature lying within the range of 80 to 175 F. In general, the temperature may be maintained at any point between room temperature and the boiling point of water but temperatures of around F. arefound very satisfactory.

From` the foregoing description of my invention', it willbe apparent that I have provided a novel composition of electrolyte and .a method whereby the saine may be used'to produce highly lustrous polishes on stainless steels, rustless iron and the like. My electrolyte and method avoid the disadvantagesof the old methods of mechanically polishing and enable the production of stainless steel and rustless iron articles having surfaces that are free from mechanical strain, dragging and pi1ing, and which are superior .to those obtainable by mechanical polishing methods.

It will, of course, be understood that .various detailsl of the process may be varied through a Wide range withoutdeparting from the principles of this invention and it is, therefore, not the purpose to limit the patent granted hereon otherwise than necessitated by the scope of the appended claims.

I claim as my invention.:

l. The method of anodically polishing stainless steel, which comprises making the stainless steel the anode in a solution comprising as -predominant ingredients, fromj0.1 to 85% of phosphoric acid, from 0.1 to 65% of CrOz, and from 13 to 66% of water, all of the percentages being by weight of said solution and the relative percentages of said three solution ingredients lying within the area dened on the accompanying diagram by the line AB, the straight lineBC, the line CD- and the straight line DA, and passing an electric current therethrough of suflicient density and for a sufllcient period of time to eiect a. polishing -of said stainless steel.

2,., The method of anodically polishing stain,

less steel, which comprises making the stainy less .steel the anode in a. solutioncontaining, as .predominant ingredients, .'from 0.1 to 85% of phosphoric acid, from 0.1 to 65% of CrOa, 'and from 13 to 47% ofv water,`all of the percentages being by weight of said solution 4and the relative percentages of said three solution ingredientsv lying within the area defined in the accompany- I ing diagram bythe line AB, the straight line BF,

the line FE and the straight line EA, and passing anelectric current therethrough of suilicient density and for a sumcient length of time to effect a polishing of said stainless steel.

3. The method of anodically polishing stainless steel, which comprises making the stainless line FE and the straight line EA, and passing an electric current therethrough of between 100 and 1,000 amperes per square foot at a bath temperature between 80 and 175 F. for a suflicient period of time to effect a polishing of said stainless steel.

` CHARLES L. FAUST. 

